So remember:
Do it in front of a surface you can see reflection of the indicators' flash, as the dash indicators didn't light up.
The initial 6 turns from LOCK to ON had to be done quickly
When doing the 6 turns, make sure you push right through to ON and not one click too short
I couldn't just register my one problem key, I had to do both.
This manifest itself in the "problem" key starting to work fine but then the "working" key stopping working. So once you get into registration mode, make sure you register both/all keys, even if one of them was already working fine.
After the first key was successfully registered (upon release of UNLOCK) I did not notice an indicator flash, as suggested in the process. I carried on regardless, took the first key out and inserted the second key for registration.
Don't forget to turn the key to ON after you have registered the last key, to go out of registration mode